1. Field of the Invention
The present invention relates to a ceramic body used as a catalyst support in an exhaust gas purifying catalyst of an automobile engine or the like, and a ceramic catalyst body.
2. Description of the Related Art
Cordierite, that has high durability against thermal shock, has been widely used as a ceramic body for a catalyst support. The catalyst is made by applying xcex3-alumina coating to the surface of cordierite that is formed in honeycomb shape, and by providing it with a noble metal catalyst supported thereon. The coating layer is formed because the specific surface area of the cordierite is too small to support a required amount of catalyst component. Thus the surface area of the support is increased by using xcex3-alumina that has a large specific surface area.
When the surface of the support is coated with xcex3-alumina, however, the heat capacity of the support increases due to the increase in the mass. Recently, investigations have been conducted to find the means to decrease the heat capacity by making the cell wall of the honeycomb support thinner, in order to achieve earlier activation of the catalyst. However, the effect of this attempt is reduced by the formation of the coating layer. There have also been such problems that the coefficient of thermal expansion of the support becomes larger due to the presence of the coating layer, and that the decrease in the opening area of the cell increases the pressure loss.
Various researches have been conducted to achieve ceramic bodies capable of supporting catalyst components without forming a coating layer. For example, Japanese Examined Patent Publication (Kokoku) No. 5-50338 proposes a method that increases the specific surface area of cordierite itself by applying heat treatment after pickling process. However, this method has not been practical because a pickling process or a heat treatment causes the destruction of the crystal lattice of cordierite, thus resulting in lower mechanical strength.
Accordingly, the present invention aims at solving the problems of the prior art described above, and providing a ceramic body capable of supporting, without forming a coating layer, a required amount of catalyst component, without lowering characteristics such as mechanical strength, thereby to provide a high performance ceramic catalyst that is excellent in practical utility and durability.
A first aspect of the invention is a ceramic body that is capable of supporting catalyst components directly on the surface thereof, and comprises a first component that constitutes a substrate ceramic and a second component that is different from the first component, wherein the second component is dispersed at least in a skin portion of the substrate ceramic.
The ceramic body of the present invention is made capable of supporting catalyst components directly thereon by dispersing the second component, that is different from the first component, in at least the skin portion of the substrate ceramic. Consequently, the problem of destruction of the crystal lattice resulting in lower mechanical strength does not occur, unlike the prior art that increases the specific surface area of the substrate ceramic by eluting the constituent components by a pickling process or the like. As a result, the ceramic body can directly support the catalyst component while maintaining a sufficient strength, and is excellent in practical utility and durability, without forming a coating layer.
Specifically, the catalyst component is supported on the second component or in an interface between the first component and the second component. When a compound including an element, that has higher strength of bonding with the catalyst component than the first component, is introduced as the second component, for example, the catalyst component can be directly supported with a strong adsorbing force. The interface between the first component and the second component includes a lattice mismatch that may generate such defects as a kink or a plate. Such a portion involves a dangling bond that can easily result in bonding with the catalyst component. As a result, as the catalyst supporting performance becomes higher and the catalyst component can be dispersed more uniformly in the support than in the case of the conventional support structure where catalytic metal particles are supported in pores, the catalyst component is less likely to coagulate and deteriorate over a long period of use.
For the first component that makes the substrate ceramic, ceramic materials such as cordierite, Al2O3, SiC, TiO2, MgO, Si2N4, ZrO2, CeO2, or SiO2 may be used. Depending on the application and the characteristics of the substrate ceramic required by the operating environment, one or several kinds selected from the ceramic materials mentioned above may be used.
The content of the second component in the ceramic body as a whole is set so that total number of atoms of metal elements that constitute the second component falls within a range from 0.1 to 70 atomic % of the total number of atoms of metal elements that constitute the first component and the second component. With the second component dispersed in the ceramic body as a whole in a proportion within the range described above, it is possible to support the required quantity of catalyst component while maintaining the characteristics of the substrate ceramic.
The content of the second component in the skin portion is preferably set so that total number of atoms of the metal elements that constitute the second component falls within a range from 0.1 to 100 atomic % of the total number of atoms of the metal elements that constitute the first component and the second component. With the content of the second component made higher in the skin portion, it is made possible to support a greater quantity of catalyst component while maintaining the characteristics of the substrate ceramic.
Moreover, the second component is preferably a compound of one or more elements having d or f orbits in the electron orbits thereof, or a composite compound of a metal element included in the first component and one or more elements having d or f orbits in the electron orbits thereof. Since elements having d or f orbits have energy levels close to that of the catalyst component, they easily donate electrons so as to form bonding. The second component may also be a compound of element having d or f orbit and the metal element included in the first component.
Furthermore, the second component may also be a compound of one or more element selected from among W, Co, Ti, Fe, Ga and Nb, or a composite compound of the metal element included in the first component and one or more element selected from among W, Co, Ti, Fe, Ga and Nb.
The compound or the composite compound mentioned above is preferably one or more kind selected from among WO3, MgWO4, CoWO4, Mg2TiO5, MgTiO3, Mg2TiO4, MgSiO3, MgWO4, MgAl2O3, TiO2, FeWO4, MgFe2O4, FeAlO3, Fe2SiO4, MgAl2O4, Al2TiO5, GaAlO3, Nb2WO3, and AlNbO4.
The mean particle size of the second component is preferably 50 xcexcm or less. Dispersing the second component having a small particle size densely in the skin portion of the ceramic body enables it to increase the quantity of catalyst component supported therein.
The shape of the ceramic body may be selected from among various shapes such as honeycomb, foamed block, hollow fiber, fiber, powder or pellets. With any such shape, it is made possible to directly support the catalyst component by adding the second component.
A second aspect of the invention is a ceramic catalyst made by directly supporting the catalyst component on the ceramic body described above. As the ceramic catalyst supports the catalyst component directly thereon and does not need a coating layer, there occurs no increase in the thermal capacity and in thermal expansion coefficient due to coating layer. The catalyst can also be activated earlier and has high durability.
Moreover, in the ceramic catalyst of the present invention, the catalyst component is supported on the second component or in the interface between the first component and the second component. When the second component includes an element that has d or f orbit in the electron orbits thereof, for example, bonding with the catalyst component is made easier. The interface between the first component and the second component is likely to involve a dangling bond formed therein, which also makes it easier to form bonding with the catalyst component.
Furthermore, the ceramic catalyst of the present invention employs the catalyst that includes noble metal as the catalyst component. Specifically, one or more element selected from among Pt, Rh, Pd, Ru, Au, Ag, Ir and In may be used according to the purpose.
In the ceramic catalyst of the present invention, the mean particle size of the catalyst component is preferably set to 100 nm or less. The smaller the particle size, the more closely the catalyst component can be dispersed, resulting in an improved catalyst performance.